Data Methodology
Where our data comes from
Every detection on HailNearMe originates from the United States federal radar network. No third-party estimates. No prediction models. Here is exactly what we use and how.
Primary Source
NOAA NEXRAD Dual-Polarization Radar
The United States operates 160 NEXRAD (Next Generation Radar) stations under the joint management of the National Weather Service (NWS), Federal Aviation Administration (FAA), and Department of Defense. These are S-band Doppler radars equipped with dual-polarization technology, meaning they transmit and receive both horizontal and vertical electromagnetic pulses.
Dual-polarization provides significantly more accurate hail detection than legacy single-pol systems. By comparing the differential reflectivity (ZDR) and correlation coefficient (ρHV) between the two polarizations, the radar can distinguish hailstones from rain droplets with high confidence — even when hail and rain are mixed in the same radar cell.
All NEXRAD data is published as federal open data under the NOAA Open Data Dissemination program. It is in the public domain with no licensing restrictions.
Data Aggregation
NOAA Severe Weather Data Inventory (SWDI)
Raw NEXRAD output is processed by NOAA's Severe Weather Data Inventory (SWDI), which applies the Multi-Year Reanalysis of Remotely Sensed Storms (MYRORSS) algorithm and the newer MRMS (Multi-Radar Multi-Sensor) system to derive hail size estimates from the raw radar reflectivity fields.
The output — the nx3hail dataset — provides point-level hail detections with associated size estimates in inches, timestamps, and geographic coordinates. HailNearMe ingests this dataset continuously and indexes it into a PostGIS spatial database for sub-second address lookup.
Hail size is a direct radar measurement, not a polygon estimate. The size reported by HailNearMe comes from the NEXRAD Mesocyclone/Hail Detection Algorithm's MAXSIZE field — a value derived directly from dual-polarization radar returns using the Maximum Expected Size of Hail (MESH) algorithm. This is distinct from the hail size mentioned in NWS severe weather warning text, which is typically a spotter estimate or modeled forecast value.
Limitations
Radar resolution and what it means for your address
NEXRAD radar gates have a range resolution of approximately 250 meters and an azimuth resolution of approximately 1 degree. At a typical storm range of 100 km from the nearest radar station, this corresponds to an effective ground resolution of roughly 1 km × 1.7 km per radar cell.
What this means practically: when HailNearMe reports hail at your address, it means a radar cell centered near your property registered a hail signature. It does not mean the radar observed your specific roof. Properties within the same ~1 km² radar footprint will all show the same detection result.
Radar cell size
~1 km²
At typical storm range
Minimum hail size
1.0 inch
Quarter-size threshold
History window
90 days rolling
From detection date
Update frequency
Within hours
After storm event
A "miss" result on HailNearMe does not certify that hail did not fall at your address. Very localized, low-altitude hail events can fall between radar sweeps. Conversely, a "hit" result reflects a radar detection in your vicinity — physical damage must be confirmed by inspection.
Ground Truth
Spotter reports and ground-truth calibration
Radar-derived hail sizes are estimates. The true size of hailstones at ground level is affected by wind drift, melting during descent, and radar beam overshooting in high-altitude convective cells. Community spotters who physically measure and photograph hailstones provide critical calibration data.
HailNearMe's spotter network accepts verified photo submissions with reported sizes. Approved spotter reports are indexed alongside NEXRAD data and are weighted in cases where radar and ground observation differ. Spotter data is labeled separately so users can distinguish between radar-derived and physically-observed detections.
Learn about our spotter networkTrajectory Analysis
Where hail actually lands
NEXRAD radar detects hail at altitude — the beam scans at an elevation angle above the horizon, not straight down. At a typical storm range of 100 km, the beam intersects the storm cell several kilometers above the ground. The hailstones detected there are still falling.
During descent, hailstones drift horizontally with the wind. A hailstone detected at 4 km altitude falling at terminal velocity through a 25 mph wind field will travel roughly 1–2 km before it reaches the ground. HailNearMe's trajectory model uses NEXRAD beam geometry, upper-air sounding data, and a physics-based fall model to estimate where that hail actually impacted — not just where the radar saw it.
Example — Trajectory Diagram
Illustrative · not real dataThe trajectory diagram is included in Radar Evidence Reports where the radar record contains azimuth and range data (most detections). It shows the detection altitude, estimated wind-drift distance, and the projected ground-impact point — which may differ from the raw radar node coordinates by up to several kilometers.
Privacy & Data Use
How we use your data
When you enter an address on HailNearMe, we geocode it to coordinates and run a spatial query against the NEXRAD dataset. Lookup data may be retained to improve the service, understand storm impact patterns, and — where you have indicated interest in an inspection or report — to connect you with licensed contractors in the StormSnipe network.
Spotter report submissions include your email address (for verification), your approximate location, and any photos you upload. This data is retained as part of the ground-truth spotter dataset. Photos may be reviewed by HailNearMe staff and may be displayed publicly as part of storm records. Submitting a report signals that you were present during a hail event — this may be used to match you with local inspection and repair resources.
Radar Evidence Report orders are processed by Stripe. HailNearMe retains your email and address to fulfill the report delivery and may follow up with relevant contractor referrals through the StormSnipe network. You can opt out at any time.